Gas sensor arrangements typically comprise a gas measuring chamber filled with a gaseous analyte, which contains at least one analyte to be measured. First and second radiation sources emit radiation toward a detector that detects the radiation and produces a mostly electric output signal, which depends on the presence and/or on the concentration of the analyte in the gas measuring chamber. The radiation sources are typically broadband radiation sources, such as incandescent lamps. These gas sensor arrangements are known for detecting the most varied types of analyte, for example, methane or carbon dioxide.
Examples of gas sensor arrangements are shown in EP 0616207 A2, WO 00/55603 A1 and DE 19925196 C2. These gas sensor arrangements are based on the concept that many polyatomic gases absorb radiation, in particular within the infrared wavelength range. This absorption occurs in a wavelength which is characteristic of the relevant gas, for example, at a wavelength of 4.24 μm in the case of carbon dioxide. The wavelength which is of interest is selected via an interference filter or grid. It is thus possible, using such gas sensor arrangements, to determine the presence of the analyte and/or the concentration of the analyte. The radiation intensity measured by the detector is a measurement of the concentration of the analyte according to the Beer-Lambert Law. This type of radiation generation is also referred to as a non-dispersive method or a non-dispersive infrared (NDIR) method in the case of an infrared-carbon dioxide analysis.
The detection of carbon dioxide is becoming increasingly important for applications in construction and automotive engineering. For example, in the case of heating and air conditioning, the content of the inside air is monitored so that when the concentration of carbon dioxide in the inside air increases a supply of fresh air is induced via a ventilation control system to increase energy efficiency. Additionally, since many modem air-conditioning systems, in particular in the automotive field, use carbon dioxide as a coolant, carbon dioxide gas sensors are needed to detect carbon dioxide leaks as a result of possible defects in the system. It has also been found that carbon dioxide concentration represents a basic indicator for the quality of inside air and is therefore highly significant as a control variable for air-conditioning systems.
Particularly in the automotive field, sensors of this type, however, must satisfy the highest requirements in terms of robustness, reliability and compactability. Long-term stability, i.e., a service life of about 10 years or more, is required. Because the specification values must be observed during the entire service life of the gas sensor arrangement, problems arise when the components of the gas sensor arrangement, in particular the radiation source and the switching electronics, age.
In order to counteract this problem, it is known to provide at least two radiation sources and two detectors in the gas sensor arrangement. One of the detectors measures the analyte and the other monitors the brightness of the radiation source using a different wavelength. With the aid of the second detector, the change detected in the brightness of the radiation source may be factored into a correction calculation. This known solution, however, suffers from the disadvantage that it is relatively complex.
Another known solution is shown in DE 199 25 196 C2. This gas sensor arrangement uses at least two radiation sources and only one detector. The first radiation source is used as a measuring radiation source and is used at a rate necessary for measurement. The second radiation source is used as a reference radiation source and is used less often and only for carrying out a comparative measurement and a referencing of the first radiation source. If the second radiation source is only used to produce a comparative measurement every x intervals compared to the measuring intervals, it may be assumed that the ageing of the second radiation source, which is lower by the factor x, is mapped onto the more severely ageing first radiation source. This known solution therefore provides a so-called bum-in phase with an extended service life for reducing an increased initial ageing.
This known solution has the disadvantage, however, that the first radiation source and the associated switching electronics age more rapidly by the factor x than the second radiation source and the associated switching electronics thereof. Thus, the final service life of the entire gas sensor arrangement is restricted by the service life of the first radiation source and the associated switching electronics.